Terminal capable of substituting for control station

ABSTRACT

A terminal of the present invention comprises a control frame detection portion of detecting a control frame issued from a control station, a control frame memory portion of storing control information contained in the control frame, a substitute frame issuance determination portion of determining to issue a substitute frame when the control frame is not detected until a predetermined first time period is elapsed, and a substitute frame issuance portion of creating a frame which is the same as or equivalent to the control frame, as the substitute frame, based on the control information stored in the control frame memory portion when the substitute frame issuance determination portion determine to issue the substitute frame, and issuing the created substitute frame.

TECHNICAL FIELD

The present invention relates to a terminal whose opportunity to accessa communication medium is controlled by a control station, and moreparticularly, to a terminal capable of substituting for a controlstation.

BACKGROUND ART

A communication system which utilizes media having a problem withcommunication quality is provided with a control station for controllingmedia access of other station(s) in order to utilize media asefficiently as possible. The other station is regarded as a controlledstation, i.e., the control station and the controlled station have amaster-slave relationship. In this manner, a central control typecommunication system protocol which can improve the efficiency of theentire communication system has been developed.

When such a protocol is applied to a wireless LAN, a power linecommunication system and the like, the efficiency of media access in thecommunication system is significantly influenced by communicationquality of a control station and a controlled station which depends on,for example, a physical positional relationship therebetween.

Japanese Patent Laid-Open Publication No. H11-8585 (PatentPublication 1) discloses a terminal which operates in a control stationmode and in a controlled station mode (two operational forms) and canselect the operational forms. In a communication system described inPatent Publication 1, the terminal detects the operating modes ofsurrounding terminals after being powered on, and when there is nodetected terminal which is activated as a control station, operates as acontrol station.

In a communication system having a control station, it is necessary toconsider the possibility that the control station gets out of orderduring operation. Further, in a power line communication system or thelike, it is considered that a station is repeatedly connected anddisconnected. When a control station is disconnected from thecommunication system, it is necessary to, for example, reconstruct acontrol station in order to achieve efficient media access. However, inthe technology of Patent Publication 1, it is not possible toreconstruct a control station, so that the communication quality of acommunication system cannot be maintained.

Japanese Patent No. 2563935 (Patent Publication 2) proposes a conceptthat a control station transfers its information to a controlled station(another station) during operation, and when the control station issuddenly disconnected from a network, the controlled station quicklyreplaces the control station to maintain network quality (PatentPublication 2).

However, in a network employing a medium, such as a radio wave or apower line, in which the reliability of transmission characteristicsvaries significantly depending on a physical positional relationshipbetween stations, the location of a control station has a significantinfluence on the transmission efficiency of the whole network.Therefore, unless a control station is determined, taking intoconsideration the location on a network of connection of a terminal, theperformance of a terminal, or the like, there is a possibility that thecommunication quality is significantly impaired upon replacement.

In a network in which importance is particularly placed on thetransmission efficiency and the Quality of Service (QoS) guaranteerequired by an application, a procedure for changing control stationsneeds to be sufficiently prepared in advance. Furthermore, a controlstation may be accidentally disconnected before giving information asdescribed in Patent Publication 2. In this case, it may be difficult toreconstruct a control station using latest information, a controlrequest which has been once accepted may be lost so that latestinformation cannot be inherited, or a network system itself may notfunction.

A terminal which operates as a control station is often designed to beconnected to a location suitable for control of a network, taking theimportance of the function thereof into consideration. Also, it is oftenthat a control station is capable of recovering itself quickly if it isdisconnected from a network for some reason. Therefore, when a controlstation is disconnected from a communication system, in general it isnot always satisfactory to replace the control station with a new one.It is often satisfactory that the original control station returns to amedia access control.

DISCLOSURE OF THE INVENTION

Therefore, a first object of the present invention is to provide aterminal having a guaranteeing mechanism such that, when an operatingcontrol station is disconnected from a communication system for somereason, media access of the communication system can be prevented fromfalling into an inefficient situation for a long time, without suddenreplacement with a new control station. Further, a second object of thepresent invention is to provide a terminal having a mechanism such that,when a control station which has been disconnected from a communicationsystem is recovered, a media access control is returned to the originalcontrol station. Furthermore, a third object of the present invention isto a terminal which can maintain the communication quality of a wholecommunication system using a simple mechanism against temporarydisconnection of a control station.

To solve the problems, the present invention provides a terminal whoseopportunity to access a communication medium is controlled in accordancewith a control frame issued from a control station connected to anetwork, comprises a control frame detection portion of detecting thecontrol frame issued from the control station, a control frame memoryportion of storing control information contained in the control frame, asubstitute frame issuance determination portion of determining to issuea substitute frame when the control frame detection portion does notdetect the control frame until a predetermined first time period iselapsed, and a substitute frame issuance portion of creating a framewhich is the same as or equivalent to the control frame, as thesubstitute frame, based on the control information stored in the controlframe memory portion when the substitute frame issuance determinationportion determines to issue the substitute frame, and issuing thecreated substitute frame.

Preferably, the control frame memory portion may store the control frameonly when the control frame has a change.

Preferably, the substitute frame issuance portion may transmit a refusalresponse frame indicating that a request which is transmitted fromanother terminal and should be responded to by the control station isrefused, during issuance of the substitute frame periodically.

Preferably, the substitute frame issuance portion may not respond to arequest which is transmitted from another terminal and should beresponded to by the control station, during issuance of the substituteframe periodically.

Preferably, the substitute frame issuance portion may stop issuing thesubstitute frame during issuance of the substitute frame periodically,if receiving a frame from the control station.

Preferably, the substitute frame issuance portion may determine whetheror not a frame is received from the control station until apredetermined second time period is elapsed. The terminal may furthercomprise a control station mode portion of causing the terminal tooperate as a control station when the substitute frame issuance portiondetermines that a frame is not received from the control station untilthe predetermined second time period is elapsed.

Preferably, the control station mode portion may collect informationrequired as a control station from another terminal.

Preferably, the control station mode portion may issue a reset signal tocollect information required as a control station from another terminal.

Preferably, when the control frame is not detected until the first timeperiod is elapsed, the terminal may try to access the communicationmedium in accordance with control information contained in a previouslyreceived control frame.

Preferably, the substitute frame issuance portion may compete for aright to access the communication medium before issuing the substituteframe, and when acquiring the access right, issues the substitute frame.

Preferably, a candidate terminal which issues the substitute frame maybe designated by the control station. The candidate terminal may begiven a priority. A terminal having a higher priority may have a higherprobability of acquiring the access right in the access rightcompetition for issuance of the substitute frame.

Preferably, a candidate terminal which issues the substitute frame maybe designated by the control station. The terminal may further comprisea candidate terminal designation recognition portion of recognizingwhether or not the control station designates the terminal as thecandidate terminal. When the candidate terminal designation recognitionportion recognizes that the terminal is designated as the candidateterminal, the substitute frame issuance determination portion maydetermine whether or not to issue the substitute frame, assuming that atime period which is shorter than a first time period in a terminalwhich is not designated as the candidate terminal, is the first timeperiod.

Preferably, the candidate terminal may be given a priority. Thesubstitute frame issuance determination portion may use a first timeperiod having a length corresponding to the priority to determinewhether or not to issue the substitute frame.

Preferably, the candidate terminal may be designated by the controlstation based on information about a communication state of a terminalin the network.

Preferably, in a situation that the terminal is designated as acandidate terminal, when the control station designates another terminalas a candidate terminal, the designation of the terminal as a candidateterminal may be released.

Preferably, an identifier for the control station may be described inthe control frame.

Preferably, the terminal may operate as the control station when anidentifier for the terminal is described in the substitute frame.

The present invention also provides a method executed in a terminalwhose opportunity to access a communication medium is controlled inaccordance with a control frame issued from a control station connectedto a network, comprising detecting the control frame issued from thecontrol station, storing control information contained in the controlframe, determining to issue a substitute frame when the control frame isnot detected until a predetermined first time period is elapsed,creating a frame which is the same as or equivalent to the controlframe, as the substitute frame, based on the stored control informationwhen it is determined to issue the substitute frame, and issuing thecreated substitute frame.

The present invention also provides a program executed in a terminalwhose opportunity to access a communication medium is controlled inaccordance with a control frame issued from a control station connectedto a network, comprising detecting the control frame issued from thecontrol station, storing control information contained in the controlframe, determining to issue a substitute frame when the control frame isnot detected until a predetermined first time period is elapsed,creating a frame which is the same as or equivalent to the controlframe, as the substitute frame, based on the stored control informationwhen it is determined to issue the substitute frame, and issuing thecreated substitute frame.

The present invention also provides a computer-readable recording mediumstoring a program executed in a terminal whose opportunity to access acommunication medium is controlled in accordance with a control frameissued from a control station connected to a network. The programcomprises detecting the control frame issued from the control station,storing control information contained in the control frame, determiningto issue a substitute frame when the control frame is not detected untila predetermined first time period is elapsed, creating a frame which isthe same as or equivalent to the control frame, as the substitute frame,based on the stored control information when it is determined to issuethe substitute frame, and issuing the created substitute frame.

Hereinafter, effects of the present invention will be described.According to the present invention, when a control station which playsan important role in maintaining communication quality is disconnectedfrom a communication system for some reason, a substitute frame which isthe same as or equivalent to a control frame previously transmitted, istransmitted. Therefore, communication quality which is the same as thatat the time of the disconnection is maintained, and in addition, it ispossible to avoid a situation that media access to the communicationsystem falls into an inefficient state for a long time.

Further, when an original control station resumes communication within apredetermined time period, issuance of the substitute frame is stopped.Thereby, the original control station can be recovered.

Also, when the original control station does not resume communicationafter the predetermined time period is elapsed, a new control stationstarts operating, thereby making it possible to avoid a situation that acontrol station is absent for a long time period.

A terminal which issues a substitute frame transmits a refusal responseframe in response to are quest which is transmitted from anotherterminal and should be responded to by a control station, or does notrespond to it. Therefore, the terminal which issues a substitute framedoes not necessarily require a function as a control station. Note thatthe terminal which issues a substitute frame is provided with a functionas a control station so as to prepare a situation that a control stationwould not be recovered. It is useful to previously designate such aterminal as a candidate terminal which issues a substitute frame.

Thus, a terminal having a simple mechanism and capable of maintainingcommunication quality of an entire communication system is provided,thereby making it possible to minimize a decrease in communicationquality in a communication system.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically showing a structure of a wholecommunication system 1 according to a first embodiment of the presentinvention.

FIG. 2 is a timing chart for explaining an exemplary operation in thecommunication system of FIG. 1.

FIG. 3 is a block diagram showing a hardware structure of a terminal 100which functions as a controlled station.

FIG. 4 is a block diagram showing a functional structure of the terminal100 which is executing a substitution control program.

FIG. 5 is a flowchart showing an operation of the terminal 100 when thesubstitution control program is executed.

FIG. 6 is a diagram showing a portion of a communication system 2according to a second embodiment of the present invention.

FIG. 7 is a flowchart showing an operation of a terminal 200 when newlyparticipating in the communication system 2.

FIG. 8 is a flowchart showing an operation of a control station.

FIG. 9 is a flowchart showing an exemplary operation of a controlstation.

FIG. 10 is a diagram showing an exemplary form of a frame when informingcontrol required information.

FIG. 11 is a timing chart showing a flow of a terminal m issuing asubstitute frame, assuming that a terminal p is disconnected from thecommunication system 2.

FIG. 12 is a block diagram showing a functional structure of theterminal 200 of the second embodiment when the substitution controlprogram is executed.

FIG. 13 is a flowchart showing an operation of the terminal 200 when asubstitution control program is executed.

FIG. 14 is a diagram showing transmission slots for a substitute framewhich depend on priorities of candidate terminals.

FIG. 15 is a block diagram shows a functional structure of a terminal300 of a third embodiment according to the present invention whenexecuting a substitution control program.

FIG. 16 is a diagram showing an exemplary format of a control frame(substitute frame).

FIG. 17 is a diagram showing another exemplary format of a control frame(substitute frame).

FIG. 18 is a diagram showing a band which is allocated for frametransmission.

FIG. 19 is a flowchart showing an operation of a terminal which isissuing a substitute frame periodically.

FIG. 20 is a flowchart showing an operation of a terminal which was acontrol station, when it is connected again to and recovered in acommunication system.

FIG. 21 is a flowchart showing an operation of a terminal afterdetecting that a control station is disconnected from a communicationsystem and determining that the terminal will function as a controlstation.

FIG. 22 is a diagram showing a whole structure of a system in which aterminal of the present invention is applied to high-speed power linecommunication.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. In a communication systemdescribed in the embodiment below, a central control type communicationsystem protocol in which media access of each terminal is controlled bya control station is employed.

First Embodiment

FIG. 1 is a diagram schematically showing a structure of a wholecommunication system 1 according to a first embodiment of the presentinvention. The communication system 1 comprises a control station and acontrolled station which is under a control of the control station. InFIG. 1, a terminal A is a control station which issues a signal forcontrolling an opportunity of media access by a terminal whichparticipates in the communication system 1 (hereinafter referred to as acontrol frame). Terminals B, C and D are controlled stations. Thecontrol frame is also called a beacon. Note that the number of terminalsin the present invention is not limited to that which is shown inFIG. 1. The terminals may be communicated with one another via wirelesscommunication, wired communication or high-speed power linecommunication. A network to which the terminals are connected is notparticularly limited.

The control station A periodically issues a control frame containingcontrol information and informs the controlled terminals of the controlframe. In the control information contained in the control frame, a timewhen media access is permitted and a time interval are described foreach controlled station or each transmission queue of controlledstations. The control information further contains a start time of atime period during which a media access right is acquired bycompetition, information for controlling media access for the timeperiod during which a media access right is acquired by competition, andthe like.

The controlled stations B, C and D store contents of a control frameevery time the control frame is received. Note that the contents of thecontrol frame may be stored only when the contents of the control frameinclude a change.

When not having received a control frame issued by the control stationfor a predetermined TIME_BC_WAIT time (first time period), a controlledstation capable of operating as a control station creates a substituteframe using control information which is stored since a control framehad been received, and performs media access competition fortransmission of the substitute frame. When winning the competition, thecontrolled station starts transmitting the substitute frameperiodically. A terminal which transmits a substitute frame periodicallyis referred to as a substitution control station.

The substitute frame contains the control information which is containedin the received control frame, without modification, so that thesubstitute frame is the same as the control frame. Alternatively, thesubstitute frame contains information equivalent to the controlinformation which is contained in the received control frame, so thatthe substitute frame is equivalent to the control frame. Here, wheninformation contained in the substitute frame inherits the controlinformation which is contained in the control frame, it is expressedthat the substitute frame is the same as or equivalent to the controlframe. Information contained in the substitute frame is considered toinherit the control information which is contained in the control frame,for example, when the substitute frame is completely the same as thecontrol frame; when the substitute frame contains at least the controlinformation contained in the control frame; when the informationcontained in the substitute frame is compatible with the controlinformation contained in the control frame. When the substitute framecontains at least the control information contained in the controlframe, the substitute frame may contain other information, such as, forexample, a flag which informs that the frame is a substitute frame, aterminal identifier of a substitution control station, or the like.

FIG. 2 is a timing chart for explaining an exemplary operation in thecommunication system of FIG. 1. Hereinafter, the exemplary operation inthe communication system of FIG. 1 will be described with reference toFIG. 2. In FIG. 2, a vertically downward direction is assumed to be thepositive direction of a time axis.

Firstly, at time T21, the control station A is assumed to transmit acontrol frame 21. The control frame 21 transmitted at time T21 isreceived by the controlled stations B, C and D. The controlled stationsB, C and D store control information contained in the received controlframe. Thereafter, at time T22, the control station A is disconnectedfrom the communication system 1. Thereby, thereafter, the controlstation A no longer issues a control frame.

When a situation in which a control frame cannot be received continuesfor a TIME_BC_WAIT time 22 (first time period), the controlled stationsB, C and D execute a competition process for media access in order toissue a substitute frame. Note that the competition process is notrequired when a substitute frame is normally issued without thecompetition process. Note that each terminal has a guaranteeing functionsuch that when no control frame can be detected for the TIME_BC_WAITtime 22, the terminal operates in accordance with a latest control frameamong those which have been received.

It is assumed that the controlled station C wins a competition processperformed from time T23 to time T24. The controlled station C issues asubstitute frame 24. Thereafter, the controlled stations B and D executemedia access in accordance with the substitute frame 24 issued by thecontrolled station C.

FIG. 3 is a block diagram showing a hardware structure of a terminal 100which functions as a controlled station. The terminal 100 is, forexample, any of the controlled stations B, C and D. In FIG. 3, theterminal 100 comprises a central processing device 101, a frametransmission/reception access processing device 102, an auxiliary memorydevice 103, and a main memory device 104. Note that a control stationcan also function as a controlled station, and therefore, a terminalwhich functions as a control station has a hardware structure similar tothat of FIG. 3.

The frame transmission/reception access processing device 102 is aninterface between a communication medium and the terminal 100, andmodulates a frame to be transmitted and demodulates a received frame.The frame transmission/reception access processing device 102 mainlyexecutes processes in lower layers, such as a physical layer, an MAClayer and the like.

The auxiliary memory device 103 is a memory device, such as a ROM, aRAM, a hard disk, an optical disc or the like. The auxiliary memorydevice 103 stores firmware or software which is executed in the centralprocessing device 101. The auxiliary memory device 103 stores asubstitution control program for causing the terminal 100 to execute anoperation of the present invention. The substitution control program maybe either firmware or software. The auxiliary memory device 103optionally stores information which is provided by the centralprocessing device 101.

The main memory device 104 is a RAM or the like and stores a program orinformation in accordance with an instruction from the centralprocessing device 101.

The central processing device 101 executes a predetermined process, andcauses the main memory device 104 to read firmware or software stored inthe auxiliary memory device 103 and executes the firmware or software.The central processing device 101 also causes the frametransmission/reception access processing device 102 to transmit/receivea frame. Thus, the central processing device 101 controls operations ofthe entire terminal 100. The central processing device 101 causes theauxiliary memory device 103 and/or the main memory device 104 to storeinformation required for processing.

FIG. 4 is a block diagram showing a functional structure of the terminal100 which is executing the substitution control program. In FIG. 4, theterminal 100 comprises a frame transmission/reception portion 105, acontrol frame detection portion 106, a control frame memory portion 107,a substitute frame issuance determination portion 108, and a substituteframe issuance portion 109.

The frame transmission/reception portion 105 transmits/receives a frameto/from a communication medium. The frame transmission/reception portion105 transfers a received frame to the control frame detection portion106 and the substitute frame issuance portion 109.

The control frame detection portion 106 determines whether or not aframe received by the frame transmission/reception portion 105 is acontrol frame. When the frame received by the frametransmission/reception portion 105 is a control frame, the control framedetection portion 106 causes the control frame memory portion 107 tostore the whole control frame or control information described in thecontrol frame.

The control frame memory portion 107 stores the control informationdescribed in the control frame transferred from the control framedetection portion 106.

The substitute frame issuance determination portion 108 determineswhether or not the current control station is disconnected from thecommunication system 1, thereby determining whether or not to issue asubstitute frame. For example, when the control frame detection portion106 does not detect a control frame for a predetermined first timeperiod (e.g., TIME_BC_WAIT time), the substitute frame issuancedetermination portion 108 determines that the current control station isdisconnected from the communication system 1, thereby determining toissue a substitute frame. Although the TIME_BC_WAIT time is here used asthe first time period, the number of frame counted may be used.Specifically, the substitute frame issuance determination portion 108counts the number of frames after receiving a control frame. When thenumber of frames exceeds a predetermined value, and nevertheless, thecontrol frame detection portion 106 does not detect a control frame, thesubstitute frame issuance determination portion 108 determines that thecurrent control station is disconnected from the communication system 1,thereby determining to issue a substitute frame. In this case, a timeperiod until the number of received frames exceeds the predeterminedvalue is the first time period. Note that the first time period is notlimited to that described above and may be any time period during whichit can be determined whether or not a control section is disconnectedfrom the communication system 1.

When the substitute frame issuance determination portion 108 determinesto issue a substitute frame, the substitute frame issuance portion 109creates a frame which is the same as or equivalent to a control frameissued by a control station, as a substitute frame, based on the controlinformation stored in the control frame memory portion 107. Thesubstitute frame issuance portion 109 executes a competition process foraccessing a communication medium, via the frame transmission/receptionportion 105 in accordance with a procedure predetermined in thecommunication system 1, such as a CSMA/CA scheme or the like. When aright to access a communication medium is obtained as a result of thecompetition process, the substitute frame issuance portion 109 createsand issues a substitute frame and causes the frametransmission/reception portion 105 to transmit the substitute frame tothe communication medium.

FIG. 5 is a flowchart showing an operation of the terminal 100 when thesubstitution control program is executed. Hereinafter, the operation ofthe terminal 100 when the substitution control program is executed willbe described with reference to FIG. 5. Hereinafter, for the sake ofsimple explanation of the operation, main bodies of operation of stepsin FIG. 5 are associated with the blocks of FIG. 4. In fact, theterminal 100 which executes the substitution control program employs thehardware of FIG. 3 to execute the procedure of FIG. 5. Therefore, themain bodies of operations of the steps in FIG. 5 can be replaced withthe central processing device 101, the frame transmission/receptionaccess processing device 102, the auxiliary memory device 103, and themain memory device 104 when implemented by hardware. This replacement isobvious to those skilled in the art and will not be explained.

The operation of FIG. 5 is started by frame reception of the terminal100 as a trigger. Initially, the control frame detection portion 106determines whether or not the received frame is a control frame (stepS101). When the received frame is a control frame, the terminal 100 goesto an operation of step S102. On the other hand, when the received frameis not a control frame, the terminal 100 goes to an operation of stepS104.

In step S102, the control frame detection portion 106 compares thepreviously received control frame and the currently received controlframe to determine whether or not the currently received control frameincludes a change. Note that when an identifier which indicates that achange is made or a change has been made is provided in a control frame,the control frame detection portion 106 may determine, based on theidentifier, whether or not the control frame includes a change. When thecurrently received control frame does not include a change, the terminal100 goes to the operation of step S104. On the other hand, when thecurrently received control frame includes a change, the control framedetection portion 106 causes the control frame memory portion 107 tostore control information described in the currently received controlframe (step S103). Thereafter, the terminal 100 goes to the operation ofstep S104. Note that the control frame detection portion 106 may notdetermine whether or not there is a change between control frames andmay cause the control frame memory portion 107 to invariably storecontrol information described in a newly received control frame. Inother words, the present invention is not limited to the operation ofFIG. 5 as long as latest control information is stored in the controlframe memory portion 107.

In step S104, the substitute frame issuance determination portion 108determines whether or not the first time period is elapsed fromreception of an immediately previous control frame. When the first timeperiod is not elapsed, the terminal 100 becomes ready to receive a frame(step S109). When receiving a frame again, the terminal 100 starts anoperation from step S101. On the other hand, when the first time periodis elapsed, the terminal 100 goes to an operation of step S105. Notethat the determination of step S104 is always performed, and when thefirst time period is elapsed from reception of an immediately previouscontrol frame, the terminal 100 performs an interrupt process to executeoperations of steps S105 to S108 no matter whether or not a frame isreceived.

In step S105, the substitute frame issuance portion 109 executes acompetition process for accessing a communication medium in accordancewith a procedure predetermined in the communication system 1 (stepS105). Next, the substitute frame issuance portion 109 determines, basedon a result of the competition, whether or not a right to access thecommunication medium is acquired (step S106). When the access right isnot acquired, the terminal 100 goes to the operation of step S109. Onthe other hand, when the access right is acquired, the substitute frameissuance portion 109 creates a substitute frame which is the same as orequivalent to the immediately previous control frame, based on thecontrol information stored in the control frame memory portion 107 (stepS107). The substitute frame issuance portion 109 causes the frametransmission/reception portion 105 to transmit the created substituteframe at predetermined time intervals (typically, time intervals atwhich a control frame is transmitted), i.e., issues a substitute frameperiodically (step S108). Although it is herein assumed that thesubstitute frame issuance portion 109 creates a substitute frame afterthe competition process, the competition process may be executed afterthe substitute frame is created.

As described above, according to the first embodiment, a terminal whichfunctions as a controlled station stores control information containedin a latest control frame. When detecting that a control station isdisconnected from a communication system, the terminal creates asubstitute frame based on the stored control information and issues thesubstitute frame periodically. Therefore, even if a control station isdisconnected from a communication system, a substitute frame iscontinued to be periodically issued in place of a control frame on thecommunication system. Therefore, media access is guaranteed on thecommunication system without suddenly exchanging with a new controlstation. As a result, it is possible to avoid a situation that theabsence of a control station causes media access to be inefficient for along time on the communication system. Further, in the first embodiment,communication quality of a whole communication system can be maintainedby a considerably simple process, i.e., storing control information andissuing a substitute frame. Thus, the present invention is practicallyvery useful.

In the foregoing description, the operation of the terminal 100 of thefirst embodiment of the present invention is implemented by executingthe substitution control program. Alternatively, the terminal 10 may beconstructed by implementing the functional blocks of FIG. 4 usingdedicated hardware, each of the functional blocks executing theoperation of FIG. 5. In this case, the dedicated hardware may beincorporated in the central processing device 101 or the frametransmission/reception access processing device 102, or alternativelymay be incorporated in other portions of the terminal 100.

Second Embodiment

A terminal according to a second embodiment of the present invention isassumed to function both as a control station and as a controlledstation. A terminal which tries to newly participate in a communicationsystem determines whether to operate as a control station or as acontrolled station before participating in the communication system.

FIG. 6 is a diagram showing a portion of a communication system 2 of thesecond embodiment. As shown in FIG. 6, the communication system 2 of thesecond embodiment is provided with a security management device 2 a. Thesecurity management device 2 a authenticates a terminal 200 which triesto participate in a communication system and manages a cryptographic keyfor a link between terminals or a cryptographic key for a wholecommunication system. Note that the security management device 2 a maybe provided in a control station which issues a control frame.

A user interface station 2 c which has a user interface which imparts acertain input means to a user, is connected to the communication system2. The user interface station 2 c may be implemented by a device otherthan the terminal 200 capable of participating in the communicationsystem 2 or by the terminal 200.

The terminal 200 and the security management device 2 a are connected toeach other via a communication medium 2 b. Note that a whole structureof the communication system 2 is similar to that of the firstembodiment, and therefore, FIG. 1 is referenced.

FIG. 7 is a flowchart showing an operation of the terminal 200 whennewly participating in the communication system 2. Hereinafter, theoperation of the terminal 200 when newly participating in thecommunication system 2 will be described with reference to FIG. 7. Notethat a hardware structure of the terminal 200 is similar to that of thefirst embodiment, and therefore, FIG. 3 is referenced. The operation ofFIG. 7 may be implemented by the central processing device 101 executinga program stored in the auxiliary memory device 103, or by dedicatedhardware.

Initially, the terminal 200 executes a terminal startup process (stepS201). Next, the terminal 200 executes carrier sense (step S202). Next,based on the carrier sense, the terminal 200 determines whether or not acontrol frame is detected (step S203).

In step S203, when a control frame is detected, a control station isalready present, and therefore, the terminal 200 communicates with thesecurity management device 2 a to execute an authentication process forparticipating in the communication system 2 (step S204). A terminalwhich can participate in the communication system 2 should be explicitlypermitted for participation. Therefore, the user utilizes the userinterface station 2 c to input fixed information (serial code) forspecifying a terminal which the user wants to participate in thecommunication system 2. The information is imparted to the securitymanagement device 2 a. The security management device 2 a accumulatesthe information. In the process of step S204, the terminal 200 whichwants to participate in the communication system 2 informs the securitymanagement device 2 a of a request for participation along with a serialcode thereof. In response to this, the security management device 2 achecks the serial code. When it can be confirmed that the terminal 200is permitted for participation by the user, the security managementdevice 2 a informs the terminal 200 of information about thecryptographic key, i.e., informs the terminal 200 which wants toparticipate in the communication system 2 of authentication/permission.On the other hand, when the informed serial code cannot be found amongthe accumulated serial codes, the security management device 2 a refusesparticipation of the terminal 200 which wants to participate in thecommunication system 2. Note that a process for authenticatingparticipation of a new entrant terminal is not limited to the exampledescribed above. For example, a new entrant terminal may beauthenticated for precipitation only when a button or the like isprovided in a user interface station and an authentication request canbe received during a certain valid time period from the time when thebutton is pressed down.

In step S203, when a control frame is not detected, the terminal 200determines whether or not a TIME_INT_WAIT time is elapsed (step S205).The TIME_INT_WAIT time is a predetermined transmission time interval ofa control frame. When the TIME_INT_WAIT time is not elapsed, theterminal 200 returns to the operation of step S202. On the other hand,when the TIME_INT_WAIT time is elapsed, the terminal 200 goes to anoperation of step S206 which is a process in which the terminal 200itself is started up as a control station.

In step S206, the terminal 200 requests a station which is only one thathas a function (BSSID manager) of permitting issuance of an ID of acommunication system (hereinafter referred to as a BSSID) for the user,for issuance of a BSSID, in order to obtain the BSSID. An effective timeduring which permission of BSSID issuance can be requested is aTIME_BSSID_WAIT time. The BSSID manager function may be possessed by theuser interface station 2 c. The BSSID manager executes management forcausing a BSSID to be only one that is effective on a time axis in alogical communication system which is desired by the user. In responseto the BSSID issuance permission request, the BSSID manager permitsissuance of a BSSID unless an effective BSSID is present at the time ofthe request.

After step S206, the terminal 200 determines whether or not issuance ofa BSSID is permitted (step S207). When issuance of a BSSID is permitted,the terminal 200 sets the BSSID by itself or is informed of issuance ofthe BBSID by the BSSID manager, to use the BSSID to start issuance of acontrol frame (step S209).

On the other hand, in step S207, when it is determined that issuance ofa BSSID is not permitted, the terminal 200 determines whether or not theTIME_BSSID_WAIT time is elapsed (step S208). When the TIME_BSSID_WAITtime is not elapsed, the terminal 200 returns to the operation of stepS206 and requests for issuance of a BSSID. On the other hand, when theTIME_BSSID_WAIT time is elapsed, the terminal 200 stops the operation(step S210). Note that the process of FIG. 7 may be repeated a pluralityof times.

As described above, by activating a control station, the communicationsystem is started up.

The control station of the second embodiment previously selects acandidate terminal which transmits a substitute frame, and gives apriority to the candidate terminal.

FIG. 8 is a flowchart showing an operation of a control station.Hereinafter, the operation of the control station will be described withreference to FIG. 8.

Initially, the control station selects a candidate terminal which issuesa substitute frame (step S301). A criterion for selecting a candidate isnot particularly limited. For example, a terminal which has a locationor performance suitable as a control station may be selected as acandidate terminal, or a terminal which is previously registered may beselected as a candidate terminal. The control station may collectinformation about a communication state of each terminal as statisticalinformation, and based on the collected statistical information, mayselect a candidate terminal.

Next, the control station gives a priority to the candidate terminal(step S302). A criterion for giving a priority is not particularlylimited. For example, a priority is given, depending on the processingperformance of a terminal, or may be randomly given without consideringthe processing performance. The control station may inform of a prioritywhich is stored in a dedicated frame or is described in a control frame.

Next, the control station transmits control required information and apriority to the candidate terminal and informs that the terminal is acandidate terminal (step S303). As used herein, the control requiredinformation refers to information which is required to operate as acontrol station. Control information contained in a control frame is aportion of control required information.

FIG. 9 is a flowchart showing an exemplary operation of a controlstation. Hereinafter, an example of selection of a candidate terminalwill be described with reference to FIG. 9. In FIG. 9, a terminal whichis currently operating as a control station is indicated by m.

Initially, the terminal m which is a control station selects a terminalp which is most appropriate to a control station, other than theterminal m, based on communication quality information of acommunication system (step S401). As used herein, the communicationquality information refers to information which indicates communicationquality of a terminal on a communication system.

Next, the terminal m which is a control station informs the selectedterminal p of information which is possessed by the terminal m and isrequired to function as a control station (control required information)(step S402). For example, the control required information includesinformation about a reserved band requested by each station in acommunication system and a request station, information for identifyinga port in use or a transmission queue, and information about a trafficsituation of a communication system.

FIG. 10 is a diagram showing an exemplary form of a frame when informingcontrol required information. The frame includes a frame header 7001 anda frame body 7002. The frame body 7002 includes a field 7003 and a field7004. The field 7003 is a field in which, when a band guarantee requestis made by a terminal in a communication system containing a controlstation in order to transmit specific stream contents, the controlstation accepts the request, and there is a link whose band isguaranteed, information about the band is described. The field 7004 is afield in which information (index) about traffic (congestion level) of awhole communication system is described. The field 7003 includes a field7005, a field 7006, and a field 7007. The field 7005 is a field whichindicates an MAC address of a request station which requests for bandallocation in a link guaranteed with a band. The field 7006 is a fieldwhich indicates information (stream ID) for specifying a transmissionqueue of the request station. The field 7007 is a field which indicatesinformation about a band for which a request is accepted. Note that aminimum band, an average band, a maximum band, a delay limit, a jitterlimit, or the like may be described in the field 7007. When there are aplurality of terminals to which bands are allocated, the data body 7002includes a plurality of fields 7003.

After step S402, the terminal m as a control station requests allterminals in the communication system 2 for release of designation as acandidate terminal (step S403). Note that it may be defined that whenone terminal in the communication system 2 is designated as a candidateterminal and other terminals are informed of control requiredinformation, the designation of the one terminal as a candidate terminalmay be released.

Next, the terminal m which is a control station determines whether ornot to change control stations by transferring the function of a controlstation to the terminal p (step S404). A criterion for determiningwhether or not to change control stations is not particularly limited.For example, when the terminal p receives a signal from other terminalsbetter than from the terminal m, the terminal m determines to changecontrol stations.

When it is determined not to change control stations, the terminal mwhich is a control station designates the terminal p as a candidateterminal for a control station (step S405). Note that when the terminalm which is a control station transmits control required information tothe terminal p, an intention to designate the terminal p as a candidateterminal may be described in the control required information. In thismanner, when one terminal is designated as a candidate terminal and acontrol station then designates another terminal as a candidateterminal, the designation of the one terminal as a candidate terminal isreleased.

On the other hand, when it is determined to change control stations, theterminal m which is the control station executes a process for causingthe terminal p to become a control station in place of the terminal m(step S406). After changing the control station, the terminal p which isa new control station creates a control frame or controls media accessin response to requests from other terminals, based on control requiredinformation imparted from the terminal m.

After step S406, the terminal m sets itself to be a candidate terminal(step S407).

Thus, when the communication system 2 has a function of changing controlstations, a candidate terminal may be determined based on whether or notto change control stations.

After the process of FIG. 9, it is currently assumed that the terminal pis a control station while the terminal m is a candidate terminal. FIG.11 is a timing chart showing a flow of the terminal m issuing asubstitute frame, assuming that the terminal p is disconnected from thecommunication system 2.

The controlled terminal of the second embodiment stores controlinformation contained in a control frame issued by a control station asdoes the controlled terminal of the first embodiment.

It is assumed that the terminal p which is a control station issues acontrol frame 31, and thereafter, is disconnected from the communicationsystem 2 without issuing a next control frame.

When a TIME_NOMINATE_WAIT time (first time period) 32 is elapsed from atime of detection of the control frame 31 issued by the terminal p, theterminal m which is a candidate terminal recognizes that the controlstation is disconnected from the communication system 2. On the otherhand, when a TIME_INIT_WAIT time (first time period) 35 is elapsed fromthe time of detection of the control frame 31, a non-candidate terminalrecognizes that the control station is disconnected from thecommunication system 2. The TIME_INIT_WAIT time is longer than theTIME_NOMINATE_WAIT time. In other words, a first time period which isused by a terminal designated as a candidate terminal is shorter than afirst time period which is used by a terminal not designated as acandidate terminal. Note that the first time period may be defined usingthe number of frames counted, as in the first embodiment.

When recognizing that the control station is disconnected from thecommunication system 2, the terminal m uses the stored controlinformation to create a substitute frame and executes a competitionprocess 33 for communication medium so as to issue the substitute frame.

After the competition process 33, when acquiring an access right, theterminal m transmits a substitute frame 34.

FIG. 12 is a block diagram showing a functional structure of theterminal 200 of the second embodiment when the substitution controlprogram is executed. In FIG. 12, the terminal 200 comprises a frametransmission/reception portion 205, a control frame detection portion206, a control frame memory portion 207, a substitute frame issuancedetermination portion 208, a substitute frame issuance portion 209, anda candidate terminal designation recognition portion 210.

The frame transmission/reception portion 205 transmits/receives a frameto/from a communication medium. The frame transmission/reception portion205 transfers the received frame to the control frame detection portion106, the candidate terminal designation recognition portion 210, and thesubstitute frame issuance portion 209.

The control frame detection portion 206 has a function similar to thatof the control frame detection portion 106 of the first embodiment. Thecontrol frame memory portion 207 has a function similar to that of thecontrol frame memory portion 107 of the first embodiment.

The candidate terminal designation recognition portion 210 recognizeswhether or not the terminal 200 is designated as a candidate terminal bya control station. When terminal 200 is designated as a candidateterminal, the candidate terminal designation recognition portion 210memorizes the designation as a candidate terminal and a prioritythereof.

The substitute frame issuance determination portion 208 referencescontents stored in the candidate terminal designation recognitionportion 210 to determine whether or not the terminal 200 is designatedas a candidate terminal. When the terminal 200 is designated as acandidate terminal, the substitute frame issuance determination portion208 determines whether or not a current control station is disconnectedfrom the communication system 1, thereby determining whether or not toissue a substitute frame. A method for determining whether or not toissue a substitute frame is similar to that of the substitute frameissuance determination portion 108 in the first embodiment.

When it is determined that the substitute frame issuance determinationportion 208 should issue a substitute frame, the substitute frameissuance portion 209 creates a frame which is the same as or equivalentto a control frame issued by the control station, as a substitute frame,based on control information stored in the control frame memory portion207. Note that the substitute frame issuance portion 209 may create asubstitute frame using control required information which has beenpreviously received from the control station.

FIG. 13 is a flowchart showing an operation of the terminal 200 when thesubstitution control program is executed. Hereinafter, the operation ofthe terminal 200 when the substitution control program is executed willbe described with reference to FIG. 13. Note that a main body of theoperation of FIG. 13 is similar to that of the first embodiment.

The operation of FIG. 13 is started by frame reception by the terminal200 as a trigger. Operations of steps S501 to S503 are similar to stepsS101 to S103 are similar to those of the first embodiment.

In step S504, the substitute frame issuance determination portion 208determines whether or not the terminal 200 is designated as a candidateterminal with reference to the candidate terminal designationrecognition portion 210. Next, the substitute frame issuancedetermination portion 208 recognizes a priority as a candidate terminal(step S505). Next, the substitute frame issuance determination portion208 determines whether or not the control station is disconnected fromthe communication system based on whether or not the terminal 200 is acandidate terminal (step S504) and the priority recognized in step S505(step S506). Note that the determination of step S506 is alwaysperformed, and when it is determined that the control station isdisconnected from the communication system 2, the terminal 200 performsan interrupt process to execute operations of steps S507 to S510 nomatter whether or not a frame is received.

There are various methods for determining whether or not a controlstation is disconnected, based on whether or not a terminal is acandidate terminal and a priority.

For example, as in FIG. 11 described above, when a first time period(TIME_NOMINATE_WAIT time) is elapsed, a candidate terminal determinesthat a control station is disconnected from a communication system. Inthis case, the first time period is such that if a control frame is notreceived during this period, it is determined that the control stationis disconnected. On the other hand, when a first time period(TIME_INIT_WAIT time) which is long than the first time period of thecandidate terminal is elapsed, a terminal other than the candidateterminal determines that the control station is disconnected from thecommunication system. As a result, the candidate terminal transmits asubstitute frame with a higher probability than that of a terminal(s)other than the candidate terminal. Further, the length of the first timeperiod may be changed, depending on the priority. Specifically, thefirst time period in a candidate terminal having a high priority may beset to be shorter than the first time period of a candidate terminalhaving a low priority. In this case, a candidate terminal having a highpriority transmits a substitute frame with a high probability.

In step S506, when determining that the control station is notdisconnected, the terminal 200 becomes ready to receive a signal (stepS511). When receiving a new frame, the operation of step S501 andthereafter are started.

In step S506, when the control station is disconnected, the substituteframe issuance portion 209 executes a competition process for access toa communication medium (step S507). Note that a candidate terminalhaving a higher priority may acquire an access right with a higherprobability in the competition process of step S507.

For example, by previously determining a slot in which a substituteframe is transmitted, depending on the priority of a candidate terminal,a candidate terminal having a higher priority acquires an access rightwith a higher probability. FIG. 14 is a diagram showing transmissionslots for a substitute frame which depend on the priorities of candidateterminals. As shown in FIG. 14, slots S31, S32, . . . , S3 n fortransmitting a substitute frame are allocated successively, depending onthe priority, from time T31 when the absence of a control station isdetermined. The slot S30 for transmitting a substitute frame isallocated at an end for a terminal which is not in formed of a priority.A candidate terminal transmits a substitute frame using a slot allocateddepending on the priority. By using such a slot, a candidate terminalhaving a higher priority acquires an access right with a higherprobability and can issue a substitute frame earlier. According to thescheme of FIG. 14, collision of substitute frames can be reliablyavoided, and even when the information of a priority or the like isincomplete, a substitute frame can be transmitted. Note that the schemeincludes a method of specifying only a single terminal having a highestpriority and issuing a substitute frame from a dedicated slot.

Note that by defining the width of a contention window based on thepriority, a terminal which is given a higher priority can be caused toacquire an access right to transmit a substitute frame, with a higherprobability. For example, time is divided into unit intervals in each ofwhich is required for frame detection on media. Such a unit is herecalled a slot. A random number is calculated where its maximum is apredetermined number of slots (here called a contention window).Transmission is performed after waiting until a time corresponding tothe calculated number of slots is elapsed (a general access control). Inthis case, if a terminal having a higher priority has a smaller width ofthe contention window, the probability of acquisition of an access rightto transmit a substitute frame can be increased.

After step S507, the substitute frame issuance portion 209 determineswhether or not an access right to transmit a substitute frame isacquired as a result of a competition process (step S508). When theaccess right is not acquired, the terminal 200 goes to an operation ofstep S511. On the other hand, when the access right is acquired, thesubstitute frame issuance portion 209 creates a substitute frame whichis the same as or equivalent to an immediately previous control frame,based on control information stored in the control frame memory portion207 (step S509). The substitute frame issuance portion 209 causes theframe transmission/reception portion 205 to transmit the createdsubstitute frame at predetermined time intervals (typically, timeintervals at which a control frame is transmitted) to issue a substituteframe periodically (step S510). Although it is herein assumed that thesubstitute frame issuance portion 209 creates a substitute frame after acompetition process, the competition process may be executed after thesubstitute frame is created.

As described above, according to the second embodiment, a terminal whichfunctions as a controlled station stores control information containedin a latest control frame. A control station previously selects acandidate terminal which issues a substitute frame and gives a prioritythereto. The terminal which functions as a controlled station determineswhether or not the control station is disconnected from a communicationsystem, based on the presence or absence of designation of a candidateterminal and the priority. When the control station is disconnected fromthe communication system, the terminal which functions as a controlledstation creates a substitute frame based on the stored controlinformation and issues the substitute frame periodically. Therefore,even when the control station is disconnected, a substitute frame isperiodically issued instead of a control station on the communicationsystem. Therefore, media access is guaranteed on the communicationsystem without suddenly exchanging with a new control station. By usinga candidate terminal and a priority, it is possible to, after a controlstation is disconnected, obtain high communication quality within acurrently feasible range. Further, since the probability that asubstitute frame is transmitted from a candidate terminal is increased,a substitute frame is transmitted from a terminal which is appropriateto transmission of the substitute frame. As a result, media access of acommunication system can be prevented from falling into an inefficientsituation for a long time due to the absence of a control station.

Note that the candidate terminal may only memorize that it is acandidate terminal and may not be given a priority.

In the second embodiment, a competition process is executed so as toobtain an opportunity to issue a substitute frame. Alternatively,instead of a competition process, a terminal may issue a substituteframe immediately after determining that a control station isdisconnected from a communication system. Still alternatively, aterminal may issue a substitute frame with timing with which a controlframe is originally transmitted.

Note that a station which stores a state of a communication system in anon-volatile ROM (flash ROM) (a station having a system backup function)may be present in the communication system. The non-volatile ROM may beany memory device which can hold information after being powered off.The non-volatile ROM stores an index capable of identifying a stationwhich is authenticated by a security manager to be able to participatein the communication system; an index capable of identifying a controlstation in the communication system; an index capable of identifying astation having the security manager function; and the like. When one ormore stations are disconnected from the communication system or arepowered off for some reason, information written in the non-volatile ROMof a station having the system backup function is utilized to recoverthe communication system. The information written in the non-volatileROM is used in selection of a control station and a security managerwhen a station in the communication system is started up again. Also inthe authentication task of the security management device 2 a in stepS204, the information written in the non-volatile ROM is utilized.Specifically, the security management device 2 a performs authenticationbased on the information written in the non-volatile ROM without theuser's reconfirmation.

Note that a terminal having such a non-volatile ROM may be a terminalhaving the BSSID manager function.

Third Embodiment

A whole structure of a communication system according to a thirdembodiment of the present invention is similar to that of the firstembodiment, and therefore, FIG. 1 is referenced. In the thirdembodiment, each terminal starts up either as a control station or as acontrolled station. Note that the communication system may include aterminal which starts up only as a control station and a terminal whichstarts up only as a controlled station.

A hardware structure of a terminal of the third embodiment is similar tothat of the first embodiment, and therefore, FIG. 3 is referenced.

FIG. 15 is a block diagram showing a functional structure of a terminal300 of the third embodiment when executing a substitution controlprogram. In FIG. 15, the terminal 300 comprises a frametransmission/reception portion 310, an operating mode management portion320, a controlled station mode portion 330, and a control station modeportion 340.

The frame transmission/reception portion 310 is an interface to acommunication medium. When receiving a frame, the frametransmission/reception portion 310 transfers the frame to the operatingmode management portion 320.

The operating mode management portion 320 manages whether or not theoperating mode of the terminal 300 is a control station mode or acontrolled station mode. When a terminal is operating, one of the modesis selected. The controlled station mode portion 330 and the controlstation mode portion 340 are connected to the operating mode managementportion 320. The terminal 300 uses the controlled station mode portion330 or the control station mode portion 340 in accordance with theoperating mode managed by the operating mode management portion 320.

The controlled station mode portion 330 comprises a control framedetection portion 331, a control frame memory portion 332, a substituteframe issuance determination section 333, and a substitute frameissuance portion 334. When the operating mode is the controlled stationmode, the operating mode management portion 320 transfers a frame fromthe frame transmission/reception portion 310 to the control framedetection portion 331.

The control frame detection portion 331 determines whether or not theframe from the operating mode management portion 320 is a control frame,and when it is a control frame, causes the control frame memory portion332 to store information (control information) contained in the controlframe.

The control frame memory portion 332 stores the control information.Note that, as in the first embodiment, only control informationcontained in a latest control frame may be stored in the control framememory portion 332.

The substitute frame issuance determination section 333 starts up atimer (not shown) every time a control frame is detected, to monitorintervals at which a control station issues a control frame. Thesubstitute frame issuance determination section 333 monitors the controlframe interval to determine the presence or absence of a control framewhich is expected to be issued for a predetermined time period. When acontrol frame is not issued, i.e., the timer is started up, for apredetermined time interval or more, the substitute frame issuancedetermination section 333 determines that the control station isdisconnected from the communication system. Note that the substituteframe issuance determination portion 333 may comprise a mechanism whichstarts up a counter for monitoring the number of received frames andresets the counter every time a control frame is detected. In this case,when the counter reaches its maximum count, the substitute frameissuance determination portion 333 determines that the control stationis disconnected from a communication system. The predetermined timeperiod or the maximum value of the counter is a first time period.

When the substitute frame issuance determination section 333 determinesthat the control station is disconnected from the communication system,the substitute frame issuance portion 334 creates a substitute frameusing the control information stored in the control frame memory portion332 and executes a competition process via the operating mode managementportion 320 and the frame transmission/reception portion 310. Whenacquiring an access right, the substitute frame issuance portion 334transfers a substitute frame to the operating mode management portion320, which in turn transmits the substitute frame via the frametransmission/reception portion 310 to a communication medium.Thereafter, the substitute frame issuance portion 334 issues thesubstitute frame periodically. When the terminal 300 receives a framefrom another terminal while the substitute frame issuance portion 334 isissuing the substitute frame, the substitute frame issuance portion 334determines whether or not the frame is a frame from a terminal which isa control station. When the frame is a frame from a terminal which is acontrol station, the substitute frame issuance portion 334 stopstransmitting the substitute frame. When a time for waiting for recoveryof a control station (second time period) is elapsed, the substituteframe issuance portion 334 instructs the operating mode managementportion 320 to switch the mode so that the terminal 300 operates as acontrol station. In response to this, the operating mode managementportion 320 switches the operating mode.

The control station mode portion 340 comprises an access control portion341 for scheduling media access of a terminal in a communicationnetwork, which is a key function of a control station, and a substitutestation management portion 342. Note that the substitute stationmanagement portion 342 may not be included.

The access control portion 341 collects a frame issuance request fromother terminal(s) operating in the controlled station mode in thecommunication system, and a frame issuance request in the terminal 300and allocates a transmission resource of a communication medium to theother terminal(s) or the terminal 300 in a time-division orfrequency-division manner. Information about the allocation of thetransmission resource is stored as scheduling information in a controlframe. The access control portion 341 issues the control frameperiodically. Such a centralized media access control can preventcollision of frames on a network in media in which collision detectionis difficult, thereby, for example, making it possible to guaranteetransmission quality required for the network without reducing the useefficiency of the media.

The substitute station management portion 342 previously sets andmanages a candidate terminal which issues a substitute frame. A methodfor selecting a candidate terminal is similar to that of the secondembodiment.

A memory portion 350 is a non-volatile memory device which is connectedto the access control portion 341. The memory portion 350 stores arequest for acquisition of a transmission resource from a terminal on anetwork, and information about updating of scheduling information, suchas allocation, change and erase of a transmission band depending on astate of a transmission channel. The memory portion 350 does not losethe stored information even when the power source of the terminal 300 issuddenly interrupted. Therefore, when a terminal which has beendisconnected from a communication system is recovered and started up asa control station, the terminal can reference to the information storedin the memory portion 350 to obtain information required as a controlstation. Note that the memory portion 350 may not be provided.

Hereinafter, an operation of each terminal when a communication systemoperates normally will be described with reference to FIG. 1.

It is assumed that, currently, the terminal A operates as a controlterminal while the terminals B, C and D operate as controlled terminalsin the communication system. The terminal A issues a control frameperiodically. In the control frame, access scheduling information aboutaccess to a communication medium is described. The terminals B, C and Daccess the communication medium in accordance with the access schedulinginformation described in the control frame. As a result, collision isprevented, and effective use of a network resource and guarantee ofrequested quality are achieved.

FIG. 16 is a diagram showing an exemplary format of a control frame(substitute frame). As shown in FIG. 16, the control frame comprises aframe header 40, an access scheduling information field 41, and a field43. The frame header 40 is provided to all frames issued in thecommunication system. The frame header 40 includes an identifier for adestination or sender terminal of a frame, a frame type, encryptioninformation, and the like. The access scheduling information field 41includes access scheduling information 42 which is scheduled by theaccess control portion 341. The access scheduling information 42 iscomposed of designation of a terminal, a data stream, a transmissionresource request, an access protocol (priority assignment by timedivision or frequency division, a competition-based access scheme incompliance with the CSMA/CA scheme, etc.). The access schedulinginformation field 41 is composed of the access scheduling information 42of each data stream, for example. The field 43 includes an identifierfor a terminal which has executed access scheduling. It is hereinassumed that the terminal A is a control station. Therefore, the field43 stores an identifier for the control station A. A format of asubstitute frame is as shown in FIG. 16, and information stored in thesubstitute frame is the same as that of a control frame. Therefore, thefield 43 of the substitute frame stores the identifier for the terminalA without modification.

The control frame issued by the control station A is received by acontrolled station. The frame transmission/reception portion 310 of thecontrolled station transfers the received control frame to the operatingmode management portion 320. The operating mode of the controlledstation is the controlled station mode, and therefore, the operatingmode management portion 320 transfers the received control frame to thecontrol frame detection portion 331. Thereafter, the control informationcontained in the control frame (in the example of FIG. 16, theinformation described in the frame header 40, information described inthe access scheduling information field 41, and information described inthe field 43) is stored in the control frame memory portion 332. Notethat past control information may be overwritten in the storing process.In addition, every time receiving a control frame, the substitute frameissuance determination section 333 starts up a timer (not shown) anddetermines whether or not the control station is disconnected from thecommunication system. A time for the determination (an expiration timeof the timer) is preferably set to be slightly longer than the controlframe issuance interval.

FIG. 17 is a diagram showing another exemplary format of a control frame(substitute frame). As shown in FIG. 17, a control frame (substituteframe) comprises a frame header 40, a substitute frame flag 52, and anaccess scheduling information field 41. The frame header 40 and theaccess scheduling information field 41 of the substitute frame arecompletely the same as the frame header 40 and the access schedulinginformation field 41 of the control frame. The control frame and thesubstitute frame are different from each other only in informationstored in the substitute frame flag 52. Therefore, when the format ofFIG. 17 is used, the substitute frame is considered to be equivalent tothe control frame.

In FIG. 17, the frame header 40 is assumed to include a source address51. The source address is also contained in the frame header of thecontrol frame. The source address of the control frame is an identifierfor the terminal A. The source address of the frame header of thesubstitute frame is the same as the source address contained in theframe header of the control frame. In other words, a terminal whichtransmits the substitute frame is other than the terminal A, however,the source address contained in the substitute frame is the identifierfor the terminal A.

The substitute frame flag 52 is false when the frame is a control frameand true when the frame is a substitute frame. Note that only asubstitute frame may include a substitute frame flag while a controlframe may not include a substitute frame flag.

It is here assumed that the power source of the terminal A isinterrupted for some reason, so that the terminal A is disconnected fromthe communication system. As a result, a control frame is no longerissued. Since a control frame is not issued, a controlled station cannotdetermine an opportunity to access to a communication medium. Note thatsince the control frame memory portion 332 of the terminal 300 storeslatest control information, the terminal 300 may determine anopportunity to access to the communication medium several times based onthe control information stored in the control frame memory portion 332.Thus, the terminal 300 may comprise a means for trying to access to acommunication medium in accordance with control information contained inthe previously received control frame even if a control frame cannot bereceived. Such a means can guarantee media access even when a controlframe is normally issued, however, is not normally received due to afluctuation in a transmission channel or the like.

When the first time period is elapsed after a control frame is no longerreceived, the substitute frame issuance determination section 333determines to issue a substitute frame.

A procedure for issuing a substitute frame may be the procedure of thefirst embodiment, or alternatively, a procedure in which a candidateterminal has a priority to issue a substitute frame as in the secondembodiment.

The substitute frame issuance portion 334 references latest controlinformation from the control frame memory portion 332 to create asubstitute frame. The substitute frame issuance portion 334 transfersthe created substitute frame to the frame transmission/reception portion310. The frame transmission/reception portion 310 executes a competitionprocess in accordance with a predetermined procedure, and when win thecompetition, transmits a substitute frame.

The competition process is executed by, for example, random backoffbased on carrier sense.

In this case, collision between substitute frames may not be detected.Therefore, a band for a substitute frame may be prepared. FIG. 18 is adiagram showing a band which is allocated for frame transmission. Asshown in FIG. 18, a control frame slot 61 is allocated for each systemcycle. The control frame slot 61 is a band in which only a control frameis issued. After the control frame slot 61, a substitute frame slot 62is allocated. In the substitute frame slot 62, a substitute frame isissued by random backoff based on carrier sense. A band 63 is for datacommunication. By exclusively preparing the control frame slot 61 andthe substitute frame slot 62 in all system cycles, it is unlikely thatcollision occurs successively many times so that the performance of thecommunication system itself is significantly reduced, even thoughsubstitute frames may collide several times.

The controlled station mode portion 330 of the terminal 300 may beprovided with the candidate terminal designation recognition portion 210as in the second embodiment. The substitute frame issuance determinationsection 333 may determine whether or not a control station isdisconnected from a communication system, based on whether or not theterminal is a candidate terminal, and the priority, as in the secondembodiment. The substitute frame issuance portion 334 may execute acompetition process based on the priority.

The frame transmission/reception portion 310 returns a result (successor failure) of issuance of a substitute frame via the operating modemanagement portion 320 to the substitute frame issuance portion 334. Forexample, when another terminal has succeeded in issuing a substituteframe earlier than the terminal to which the frametransmission/reception portion 310 is included, the frametransmission/reception portion 310 returns the failure of issuance of asubstitute frame to the substitute frame issuance portion 334 withouttransmitting a substitute frame. When the failure is returned, thesubstitute frame issuance portion 334 stops issuance of a substituteframe. Thereby, a sequence for issuance of a substitute frame is ended.

It is here assumed that a message indicating that issuance of asubstitute frame is succeeded is input to the substitute frame issuanceportion 334. The substitute frame issuance portion 334 continues toissue a substitute frame in accordance with transmission frame cyclesfor a predetermined time TIME_TENT or a predetermined number of timesCNT_TENT. The predetermined time TIME_TENT or a time periodcorresponding to the predetermined number of times CNT_TENT is a secondtime period. In this case, an identifier for the terminal A which is acontrol station is described in the field 43 of a substitute frame thusissued. An identifier for a terminal which issues the substitute frameis not described in the field 43. Note that an identifier for asubstitute frame is described in the frame header 40 of the substituteframe. Therefore, if the terminal A which was a control station issubsequently recovered, it can be recognized that a frame transmitted atcontrol frame cycles is a substitute frame.

Note that, when a format shown in FIG. 17 is used for a substituteframe, a terminal which issues the substitute frame cannot be identifiedfrom information stored in the substitute frame. However, the substituteframe flag 52 describes whether or not the frame is a substitute frame.Therefore, a terminal which receives a substitute frame can recognizewhether the frame is a substitute frame or a control frame, byreferencing the substitute frame flag 52. When it is recognized that theframe is a substitute frame, it may be defined that a terminal whichreceives a substitute frame does not request for a control station.

A terminal which issues a substitute frame periodically issues a framewhich is the same as or equivalent to a control frame, and does notoperate as a control station. In other words, the terminal which issuesa substitute frame does not execute band allocation or the like.Therefore, the terminal which issues a substitute frame cannot acceptanother terminal's request for allocation of a transmission resource.Therefore, the terminal which issues a substitute frame refuses allrequests from the other terminals, and creates a refusal response frameindicating refusal of the request and returns the frame to the otherterminal. Note that the terminal which issues a substitute frame mayignore all requests and may not return a response. Thus, a function ofthe terminal which issues a substitute frame is executed in thecontrolled station mode portion 330, and therefore, the terminal whichissues a substitute frame cannot still be said to be a control station.Note that a control station is considered to have various functions,such as acceptance of a new terminal onto a network (authentication,attribution, security setting, etc.). However, the terminal which issuesa substitute frame issues a refusal response frame to all request framesdirected to a control station.

When the terminal which issues a substitute frame detects a frametransmitted from the terminal A which was a control station while beingoperating as a substitute control station, the terminal which issues asubstitute frame determines that the terminal A is recovered as acontrol station and stops issuing a substitute frame.

FIG. 19 is a flowchart showing an operation of a terminal which isissuing a substitute frame periodically. Hereinafter, the operation ofthe terminal which is issuing a substitute frame periodically will bedescribed with reference to FIG. 19.

It is here assumed that the substitute frame issuance portion 334 isissuing a substitute frame periodically. The substitute frame issuanceportion 334 determines whether or not a predetermined wait time forrecovery of a control station (second time period) is elapsed (stepS601). Specifically, the substitute frame issuance portion 334determines whether or not a timer which is set to be TIME_TENT isexpired or a counter which is set to be CNT_TENT is expired. Note thatthe second time period may be longer or shorter than or the same as thefirst time period.

When the control station recovery wait time is not elapsed, thesubstitute frame issuance portion 334 determines whether or not a frameis received (step S604). When a frame is not received, the substituteframe issuance portion 334 returns to the operation of step S601. On theother hand, when a frame is received, the substitute frame issuanceportion 334 goes to an operation of step S605.

In step S605, the substitute frame issuance portion 334 determineswhether or not the received frame is a frame which is from a controlstation which has been determined to be disconnected. When the receivedframe is not a frame from a control station, the substitute frameissuance portion 334 transmits a refusal response frame in response tothe received frame, or ignores or does not respond to the received frame(step S608), and returns to the operation of step S601. On the otherhand, when the received frame is a frame from a control station, thesubstitute frame issuance portion 334 goes to an operation of step S606.

In step S606, the substitute frame issuance portion 334 stops issuing asubstitute frame. This is because it is recognized that the controlstation is recovered in the communication system. Thereafter, thesubstitute frame issuance portion 334 requests the operating modemanagement portion 320 to operate in a controlled station mode (stepS607), and ends the process.

On the other hand, when it is determined that the control stationrecovery wait time is elapsed in step S601, the substitute frameissuance portion 334 collects information required for the terminal 300to operate as a control station (control required information) fromanother terminal (step S602). Specifically, in step S602, wheninformation stored in the control frame memory portion 332 is notsufficient for an operation as a control terminal, the substitute frameissuance portion 334 transmits a request for required information toanother terminal, and collects information contained in a frame returnedfrom the other terminal to obtain information required for an operationas a control station. Note that the substitute frame issuance portion334 may transmit a reset signal to a network and collect a controlrequest from another terminal to collect information required as acontrol station.

After step S602, the substitute frame issuance portion 334 requests theoperating mode management portion 320 to operate in the control stationmode (step S603), and ends the process. When operating in the controlstation mode, a terminal which issues a substitute frame uses the field43 of a control frame as an identifier for itself. Alternatively, theterminal which issues a substitute frame uses the source address 51 ofthe frame header 40 in a control frame as an identifier for itself.

The process of step S602 may be executed either before or after theprocess of step S603. In other words, the required information may becollected after issuance of a control frame. Therefore, when theinformation required as a control station cannot be sufficientlyobtained from the control information stored in the control frame memoryportion 332, the terminal which issues a substitute frame transmits aninquiry to another terminal in the communication system while issuing acontrol frame, to collect the required information. Thereby, networkquality is maintained. Alternatively, the terminal which issues asubstitute frame may transmits an inquiry to another terminal in thecommunication system using contents of a substitute frame to collectlatest required information. Note that the terminal which issues asubstitute frame may continue to refuse a control request untilsufficient information is collected.

FIG. 20 is a flowchart showing an operation of a terminal which was acontrol station and is thereafter connected again to and recovered in acommunication system. Hereinafter, the operation of the terminal whichwas a control station and is thereafter is connected again to andrecovered in a communication system, will be described with reference toFIG. 20. The terminal which was a control station is represented by aterminal A.

Initially, the terminal A starts up in the controlled station mode (stepS701). Next, the terminal A receives a substitute (or control) frame(step S702). Note that, in a situation that the second time period iselapsed and therefore a terminal which issues a substitute frameoperates as a control station, the terminal A receives a control framerather than a substitute frame.

After step S702, the terminal A determines whether or not a terminalwhich performs scheduling of band allocation is set to be the terminal Ain the received substitute (or control) frame (step S703). Specifically,when the control frame has a formant of FIG. 16, the terminal Areferences the field 43 of the substitute frame to determine whether ornot a terminal which performs scheduling is the terminal A. Note thatwhen the control frame has the formant of FIG. 17, the terminal Areferences of the source address 51 of the frame header 40 to determinewhether or not a terminal which performs scheduling is the terminal A.

In step S703, when a terminal which performs scheduling is assumed to bethe terminal A, the terminal A operates in the control station modesince the frame received in step S702 is a substitute frame (step S705).

On the other hand, when it is determined in step S703 that the terminalwhich performs scheduling is not the terminal A, the terminal A operatesin the controlled station mode since the frame received in step S702 isa control frame which is issued by a new control station (step S704).

Note that, when operating in the control station mode, the terminal Amay transmit a particular frame for informing of recovery in thecommunication system, to a terminal which is operating in the substitutecontrol station, rather than transmitting a control frame.

When operating in the control station mode, the terminal A may issue acontrol frame by utilizing a band, such as a control frame slotallocated for a control frame or the like. Thereby, it is easier todetermine that a terminal which is issuing a substitute frame receives acontrol frame from a control station in steps S604 and S605.

When operating in the control station mode, the terminal A stores thereceived substitute frame into the control frame memory portion 332.When operating in the control station mode, the terminal A mayreconstruct a control frame using the substitute frame stored in thecontrol frame memory portion 332. However, the substitute frame isconstructed based on a control frame previously used, and therefore, maynot store control information which is suitable for a latest situation.The control information stored in the substitute frame is only a portionof information required for an operation as a control station. Forexample, usually, detailed information about requirement for atransmission resource for performing rescheduling of media access, andinformation about a terminal participating in a network (informationabout authentication, performance or the like) are not described in acontrol frame. Therefore, the terminal A collects information from otherterminal(s) to obtain control information required as a control station.

Hereinafter, a method of collecting information by the terminal A whichoperates as a control station will be described. Note that the methoddescribed below can be applied to the process in which a terminal whichis operating as a substitute control station collects information whenoperating in the control station mode (step S602 of FIG. 19).

For example, a terminal which operates as a control station uses anempty transmission band to obtain required information from a terminalon the communication system.

For example, an identifier for a terminal which manages band reservationinformation for achieve QoS must be described in a substitute framewhich is a duplicate of a control frame. Therefore, a terminal whichoperates as a control station issues a confirmation frame for confirmingband reservation information, to a terminal which has the identifier,thereby collecting information about band requirement.

For example, a terminal which operates as a control station communicatesa terminal which performs authentication management to obtainauthentication information, thereby confirming a terminal whichparticipates in the communication system.

Thus, the terminal which operates as a control station transmits aninquiry to a terminal in the communication system to collect requiredinformation.

Note that the terminal which operates as a control station may refusereception of a control request until all required information isobtained.

Thus, according to the third embodiment, when receiving a frame from anoriginal control station during the second time period, the terminalwhich issues a substitute frame periodically stops issuance of asubstitute frame and operates in the controlled station mode. Therefore,a media access control by the original control station can be recovered.As a result, an efficient media access control can be achieved. Theterminal which issues a substitute frame periodically transmits arefusal response frame or does not respond to a request from terminalsother than an original control station during the second time period.Therefore, a state when the control station was disconnected ismaintained until the control station is recovered. Therefore, when thecontrol station is recovered, a post-recovery process can be smoothlyperformed. When the control station is not recovered after the secondtime period is elapsed, a terminal which transmits a substitute framesubsequently operates as a control station, thereby eliminating asituation that a control station is absent.

A terminal which becomes a control station since an original controlstation is not recovered, collects information to obtain informationrequired as a control station if information required for a media accesscontrol is insufficient. Therefore, it is possible to avoid a situationthat media access is inefficient for a long time in the communicationsystem.

Note that a control station may inform a candidate terminal of controlrequired information or may inform each terminal of control requiredinformation by multi-broadcasting. In this case, a controlled stationcauses the control frame memory portion 332 to store control requiredinformation. When the controlled station detects that the controlstation is disconnected from the communication system since the firsttime period is elapsed and determines that it will functions as acontrol station since the second time period is elapsed, the controlledstation starts an operation shown in FIG. 21. For example, after thedetermination of step S601 in FIG. 19 is affirmative, the operation ofFIG. 21 is started.

FIG. 21 is a flowchart showing an operation of a terminal afterdetecting that a control station is disconnected from the communicationsystem and determining that the terminal will function as a controlstation. In FIG. 21, initially, the terminal determines whether or notthe terminal has been informed of latest control required information(step S801). For example, it is assumed that, when control requiredinformation includes an item which has not been updated, an identifierfor the item which has not been updated is store in a control frame,which is in turn transmitted by a control station. In this case, theterminal determines whether or not there is an item which has not beenupdated, based on the recorded control information. When there is not anitem which has not been updated, the terminal determines that theinforming of latest control required information has been completed.

When the informing of latest control required information has beencompleted, the terminal creates a control frame based on the receivedcontrol required information and starts operating as a control terminal(step S802). On the other hand, when the informing of latest controlrequired information has not been completed, the terminal starts anoperation as a control station after collecting required information asin step S602 of the third embodiment of FIG. 19 (step S803). Note that,when the latest control required information has been partly received instep S803, the terminal may start an operation as a control stationusing the partly received control required information.

Note that functional blocks required to achieve the terminal of thepresent invention may be implemented as an integrated circuit (LSI). Thefunctional blocks may be mounted on one chip, or a part or the whole ofthe functional blocks may be mounted on one chip. As used herein, LSImay be called IC, system LSI, super LSI or ultra LSI, depending on thepackaging density. The integrated circuit is not limited to LSI. Theintegrated circuit may be achieved by a dedicated circuit or ageneral-purpose processor. Further, an FPGA (Field Programmable GateArray) which can be programmed after LSI production or a reconfigurableprocessor in which connection or settings of circuit cells in LSI can bereconfigured, may be used. Furthermore, if an integrated circuittechnology which replaces LSI is developed by an advance in thesemiconductor technology or other technologies derived therefrom, thefunctional blocks may be packaged using such a technology. Abiotechnology may be applicable.

Hereinafter, an example in which the above-described embodiment isapplied to an actual network system will be described. FIG. 22 is adiagram showing a whole structure of a system in which a terminal of thepresent invention is applied to high-speed power line communication. Asshown in FIG. 22, the terminal of the present invention provides aninterface between a multimedia apparatus (e.g., a digital television(DTV), a personal computer (PC), a DVD recorder, etc.) and a power line.An interface between the multimedia apparatus and the terminal of thepresent invention may be an IEEE1394 interface, a USB interface, anEthernet(R) interface or the like. With such a structure, acommunication network system which transmits digital data, such asmultimedia data or the like, via a power line with high speed, isconstructed. As a result, a power line which is already provided inhome, offices and the like can be used as a network line without newlyintroducing a network cable, such as a conventional wired LAN.Therefore, the present invention is considerably useful in terms of costand simplicity of installation.

In the embodiment of FIG. 22, the terminal of the present inventionserves as an adaptor which converts a signal interface of an existingmultimedia apparatus to an interface of power line communication. Theterminal of the present invention may be incorporated into a multimediaapparatus, such as a personal computer, a DVD recorder, a digitaltelevision, a home server system or the like. Thereby, data transferbetween apparatuses can be achieved via a power source cable of themultimedia apparatus. In this case, a line for connecting the adaptorand the power line, an IEEE1394 cable, a USB cable, and an Ethernet (R)cable are not required, thereby simplifying wiring.

Also, a communication network system which uses a power line can beconnected via a router and/or a hub to the Internet, a wireless LAN, anda conventional wired cable LAN, thereby extending a LAN system in whichthe communication network system of the present invention is usedwithout any problem.

Communication data transferred via a power line may be intercepted by anapparatus directly connected to the power line, but is free from aneaves drop problem with wireless LAN. Therefore, the power linetransmission scheme is effective in terms of security. Further, datatransferred on a power line may be protected by IPSec of an IP protocol,encryption of the contents themselves, other DRM schemes, or the like.

In the future, it is expected that a control station is provided as aparticular terminal distinguished from a semi-fixed terminal having ageneral function, in the high-speed power line communication, if takinginto consideration coexistence of adjacent access systems, provision ofa gateway/router function, positional characteristics of a power line,expected characteristics of a vender, and the like. Therefore, when asemi-fixed control station is used, indiscriminately changing of controlstations would lead to a situation that communication quality is notguaranteed. Therefore, a system of the present invention is practicallyvery useful, in which, when a control station is disconnected from acommunication system, another terminal issues a substitute frame, andthereafter, if a control station is recovered, issuance of a controlframe is inherited to the recovered control station.

As described above, when a central control network protocol which uses acommunication medium, such as a radio wave, a power line, a telephoneline, a coaxial cable or the like, which has less reliable communicationquality, is employed to construct a home network communication system,the terminal of the present invention may be used as a home electricalproduct which is a communication module having a communication function.For a home network communication system, situations, such as the powersource of a control station which have an important influence on a wholecommunication system is abruptly shut down, a connecting point of acontrol station and a communication medium is removed, and the like, arereadily expected. In these situations, the present invention provides asubstitute control station which issues a substitute frame to keep acommunication process effective. Therefore, there is a possibility thatthe present invention provides a means for optimizing communicationquality of a communication system, and further, solving an adjacentnetwork interference problem and an adjacent channel interferenceproblem.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

INDUSTRIAL APPLICABILITY

The terminal of the present invention can avoid a situation that acommunication system falls into an inefficient state for a long time,and is useful in communication fields, such as wireless communication,power line communication and the like.

1. A terminal whose opportunity to access a communication medium iscontrolled in accordance with a control frame issued from a controlstation connected to a network, the terminal comprising: a control framedetection portion of detecting the control frame issued from the controlstation; a control frame memory portion of storing control informationcontained in the control frame; a substitute frame issuancedetermination portion of determining to issue a substitute frame whenthe control frame detection portion does not detect the control frameuntil a predetermined first time period is elapsed; and a substituteframe issuance portion of creating a frame which is the same as orequivalent to the control frame, as the substitute frame, based on thecontrol information stored in the control frame memory portion when thesubstitute frame issuance determination portion determines to issue thesubstitute frame, and issuing the created substitute frame.
 2. Theterminal according to claim 1, wherein the control frame memory portionstores the control frame only when the control frame has a change. 3.The terminal according to claim 1, wherein the substitute frame issuanceportion transmits a refusal response frame indicating that a requestwhich is transmitted from another terminal and should be responded to bythe control station is refused, during issuance of the substitute frameperiodically.
 4. The terminal according to claim 1, wherein thesubstitute frame issuance portion does not respond to a request which istransmitted from another terminal and should be responded to by thecontrol station, during issuance of the substitute frame periodically.5. The terminal according to claim 1, wherein the substitute frameissuance portion stops issuing the substitute frame during issuance ofthe substitute frame periodically, if receiving a frame from the controlstation.
 6. The terminal according to claim 1, wherein the substituteframe issuance portion determines whether or not a frame is receivedfrom the control station until a predetermined second time period iselapsed, and the terminal further comprises: a control station modeportion of causing the terminal to operate as a control station when thesubstitute frame issuance portion determines that a frame is notreceived from the control station until the predetermined second timeperiod is elapsed.
 7. The terminal according to claim 6, wherein thecontrol station mode portion collects information required as a controlstation from another terminal.
 8. The terminal according to claim 6,wherein the control station mode portion issues a reset signal tocollect information required as a control station from another terminal.9. The terminal according to claim 1, wherein, when the control frame isnot detected until the first time period is elapsed, the terminal triesto access the communication medium in accordance with controlinformation contained in a previously received control frame.
 10. Theterminal according to claim 1, wherein the substitute frame issuanceportion competes for a right to access the communication medium beforeissuing the substitute frame, and when acquiring the access right,issues the substitute frame.
 11. The terminal according to claim 10,wherein a candidate terminal which issues the substitute frame isdesignated by the control station, the candidate terminal is given apriority, and a terminal having a higher priority has a higherprobability of acquiring the access right in the access rightcompetition for issuance of the substitute frame.
 12. The terminalaccording to claim 1, wherein a candidate terminal which issues thesubstitute frame is designated by the control station, the terminalfurther comprises a candidate terminal designation recognition portionof recognizing whether or not the control station designates theterminal as the candidate terminal, and when the candidate terminaldesignation recognition portion recognizes that the terminal isdesignated as the candidate terminal, the substitute frame issuancedetermination portion determines whether or not to issue the substituteframe, assuming that a time period which is shorter than a first timeperiod in a terminal which is not designated as the candidate terminal,is the first time period.
 13. The terminal according to claim 12, thecandidate terminal is given a priority, and the substitute frameissuance determination portion uses a first time period having a lengthcorresponding to the priority to determine whether or not to issue thesubstitute frame.
 14. The terminal according to claim 11, wherein thecandidate terminal is designated by the control station based oninformation about a communication state of a terminal in the network.15. The terminal according to claim 11, wherein in a situation that theterminal is designated as a candidate terminal, when the control stationdesignates another terminal as a candidate terminal, the designation ofthe terminal as a candidate terminal is released.
 16. The terminalaccording to claim 1, wherein an identifier for the control station isdescribed in the control frame.
 17. The terminal according to claim 16,wherein the terminal operates as the control station when an identifierfor the terminal is described in the substitute frame.
 18. A methodexecuted in a terminal whose opportunity to access a communicationmedium is controlled in accordance with a control frame issued from acontrol station connected to a network, a method comprising: detectingthe control frame issued from the control station; storing controlinformation contained in the control frame; determining to issue asubstitute frame when the control frame is not detected until apredetermined first time period is elapsed; creating a frame which isthe same as or equivalent to the control frame, as the substitute frame,based on the stored control information when it is determined to issuethe substitute frame; and issuing the created substitute frame.